Yikes, looks like Crowsnest Pass is a no-go; "
You Need a Thneed" post here:
http://forum.calgarypuck.com/showpos...3&postcount=54
I took this route in March. Calgary - Whitefish - Seattle - Whistler. I drove through Fernie, and then south. That doesn't appear to be an option.
I've adjusted it to cross the border on the Alberta side.
http://goo.gl/maps/kk9dG
Good luck!